1. Field of the Invention
The present invention relates to a bladder for a vulcanizer in which, when a green tire is subjected to vulcanizing-molding, the inner wall surface of the tire is pressed in a mold direction by a heat medium. The present invention further relates to a vulcanizing-molding method for vulcanizing-molding a green tire by pressing the inner wall surface of the tire in a mold direction through the bladder, and to a vulcanizer.
2. Description of the Related Art
Conventionally, a bladder type vulcanizer is provided with a bladder formed of rubber such as butyl rubber having excellent flexibility in an expansible manner. In vulcanizing-molding a green tire, first, the bladder is contracted so as to have a smaller diameter than that of a tire bead of the green tire, the green tire is inserted into a bladder stood-up portion, the bladder is then inflated, the green tire is held so that the bladder is placed along the inner surface of the green tire, and a mold is clamped. Thereafter, a heat medium of high temperature and high pressure is supplied into the bladder to stretch the bladder into close contact with the inner wall surface of the tire, and the bladder is pressed in a mold direction while heating the inner wall surface of the tire through the bladder to thereby form a tire groove of the mold in the tread portion of the green tire. At the same time, the green tire is heated from outside and inside thereof by the heated mold and the bladder in contact with the heat medium of high temperature to vulcanize it. After passage of a fixed vulcanizing time, the mold is opened and the bladder is contracted. The vulcanized tire is then removed from the bladder and carried outside the machine, thus completing the vulcanizing-molding.
However, rubber in a conventional bladder is apt to harden at high temperatures. This leads to a decrease in pressing force, and to elongation, of the bladder with repeated use in vulcanization-molding. As a result, after a conventional rubber bladder is used a number of times to vulcanize tires, irregularities in the shape of the inner wall surface of the vulcanized tires become noticeable. The irregularities lead to lower tire quality. In order to maintain tire quality within set ranges, conventional rubber bladders are replaced after every 300-400 uses, which has greatly hindered mass production of tires.
Furthermore, when vulcanizing-molding is carried out using a bladder made of rubber, the bladder itself impedes heat transmission to a green tire when a heat medium is supplied into the bladder to heat the green tire because the thermal conductivity of rubber is low. Accordingly, it takes a long time for the green tire to heat to the vulcanizing temperature after a supply of the heat medium, thus delaying vulcanizing-molding, and resulting in a lowering of productivity.
The present invention provides a vulcanizer bladder which decreases the occurrence of irregularities in the shape of the inner wall of tires, so that the frequency of exchange of the bladder in a vulcanizer can be reduced. The present invention also provides a vulcanizing-molding method and a vulcanizer capable of completing the vulcanizing-molding in a short period of time.
The vulcanizer bladder according to the present invention is a bladder that, after being supplied with a pressurized medium, presses against the inner wall surface of a green tire towards a mold during vulcanizing-molding. The bladder has, as a constituent member, a low elongation material that has stable properties at vulcanization temperatures. The bladder is formed so that, before use, it has substantially the same shape as the inner wall surface of the vulcanized tire that results after vulcanization-molding.
In this case, since the bladder is formed of a low elongation material formed or shaped to have the shape of the inner wall surface of the vulcanized tire, the bladder is merely stretched by pressure gas whereby the shape of the inner wall surface of the vulcanized tire can be realized by the bladder. Accordingly, the green tire is pressed by the bladder for vulcanizing-molding to thereby vulcanize-mold the green tire with high accuracy. Since the low elongation material has stable properties at high temperatures, even when vulcanizing-molding is repeated, the low elongation material maintains its initial properties. Thus, even when the frequency of vulcanizing-molding increases, green tires can be vulcanized and molded with rare occurrence of irregularities in the shape of the inner wall surface of a tire. As a result, the frequency of exchange of the bladder can be reduced.
The low elongation material preferably has an elongation at 200xc2x0 C. in the range of 5% to 15%.
In this case, high-accuracy vulcanizing-molding of green tires can be realized repeatedly.
Preferably, the low elongation material is formed of knitwork or woven stuff, and comprises at least one of organic fibers such as nylon, polyester, aramid, poly-paraphenylene benzo-bis-oxazol (PBO), polyphenylene sulfide, overall aromatic polyester, etc., inorganic fibers such as glass fiber, carbon fiber, etc., metal cord material, carbon-contained fiber, metal-coated fiber, and resin-coated fiber.
In this case, various low elongation materials can be combined to provide a bladder most suitable for conditions of vulcanizing-molding.
Further, preferably, resin such as fluoride, silicone, etc., and at least one kind of elastomer is impregnated in or coated on the low elongation material.
In this case, air-tightness of the bladder can be made to approximately the same as a bladder made of butyl rubber, and it is possible to prevent the pressure medium from leaking outside the bladder.
Further, in the vulcanizing-molding according to the present invention, when a pressurized medium is supplied into the bladder, and the inner wall surface of a tire is pressed in a mold direction through the bladder, the bladder itself can act as a source of heat.
The vulcanizer according to the present invention is a vulcanizer in which the inner wall surface of a green tire charged into a mold is pressed in a mold direction and heated to vulcanize and mold the green tire. The vulcanizer comprises a bladder that can have a conductive material in at least a part thereof. The bladder can be stretched by a supply of a pressurized medium to press the inner wall surface of the green tire toward a mold while heating the tire. A power supply can supply power to the conductive (i.e. resistive) material in the bladder can heat the bladder.
In this case, since the heat generated in the bladder can be utilized for heating the inner wall surface of a tire, the inner wall surface of a tire can be heated for vulcanizing-molding without using a pressurized medium of high temperature and high pressure as in the prior art. Furthermore, when both the high temperature pressurized medium and the conductive material both supply heat to the green tire, the vulcanizing-molding can be completed in a shorter vulcanizing time.
Preferably, the conductive material comprises one or more of conductive rubber, conductive fiber, conductive resin, meal cord material and heat generating coil.
In this case, the bladder can be formed easily.
Preferably, the conductive material is concentrated in the xe2x80x9cvulcanization delay partxe2x80x9d of the bladder, which generally presses against the thicker wall portions of the green tire (mainly, portions of the tread and bead) where vulcanization is slowest in order to concentrate heat and accelerate vulcanization relative to other parts of the green tire.
In this case, since the vulcanization delay part of the bladder receives a supply of more heat than other parts, the thicker wall portion of the green tire corresponding to the vulcanization delay part can be heated rapidly. Accordingly, vulcanizing-molding of the green tire can be completed in a shorter period of time.
The vulcanizer according to the present invention is a vulcanizer in which the inner wall surface of a green tire charged into a mold is pressed in a mold direction and heated to vulcanize and mold the green tire, and comprises a bladder, which can have a conductive material in at least a part thereof and which can be stretched by a supply of a pressurized medium to press the inner wall surface of the green tire toward the mold.
In this case, it is possible to heat the bladder itself, and the heat generated in the bladder is utilized for heating the inner wall surface of a tire. Therefore, the vulcanizing-molding can be completed in vulcanizing time reduced by time required for heating the bladder, as compared with the conventional case where the pressurized medium applies the heat quantity to the bladder to heat the latter, after which the inner wall surface of a tire is heated.
Preferably, a magnetic material can be concentrated in the vulcanization delay part so that heat generation in the vulcanization delay part corresponding to the thicker wall portions of the green tire is accelerated relative to other parts.
In this case, since the vulcanization delay part of the bladder receives a supply of more heat than other parts, the thicker wall portions of the green tire corresponding to the vulcanization delay part can be heated rapidly. Accordingly, the vulcanizing-molding of the green tire can be completed in a shorter period of time.
Even where the bladder has a low elongation material as a constituent member, it may have the conductive material or the magnetic material in a part of the bladder. In this case, it is preferable in that a geometrical effect is obtained.